(1) Field of the Invention
The present invention relates to an ion source of a mass spectrometer or the like. More particularly, the present invention relates to an improvement in the chemical ionization ion source.
(2) Description of the Prior Art
The chemical ionization and equipments for accomplishing the chemical ionization are described in detail in, for example, Analytical Chemistry, 47, No. 11, pages 1730-1734, September 1975, the specification of U.S. Pat. No. 3,555,272 and the specification of Japanese Patent Application Publication No. 36190/75.
Ionization is one of the methods for ionizing a sample. The present invention relates to a soft ionization method utilizing ion-molecule reactions between reactant ions having a low energy corresponding almost to a thermal kinetic energy and the sample. According to this method, a pressure of 0.1 to several mm Hg (Torr) is maintained in an ionization chamber and in this chamber a reagent gas is ionized by electron impact, whereby ion-molecule reaction is caused between the resulting ion and a neutral reagent gas and a stable reactant ion is formed. Then, the sample is ionized by an ion-molecule reaction between this reactant ion and the sample.
FIG. 1 illustrates a mass spectrometer provided with a conventional chemical ionization ion source. In FIG. 1, reference numeral 1 represents an ionization chamber, numeral 2 represents repeller electrodes for pushing out ions formed in the ion source, numeral 3 an inlet for reagent gas and sample gas, numeral 4 a slit, numeral 5 a slit, numeral 6 a lens electrode, numeral 7 an electron gun, numeral 8 a mass analyzing region, and reference numeral 9 represents a detecting region. As is apparent from FIG. 1, in the conventional chemical ionization ion source, electrons generated by the electron gun 7 maintained under a pressure lower than 10.sup.-4 mm Hg are accelerated by a voltage of several hundred volts to about 1 KV and introduced into the ionization chamber 1 maintained under a pressure of 0.1 to several mm Hg thereby creating a pressure gradient between the electron gun 7 and the ionization chamber 1. In the ionization chamber 1 the reagent gas is ionized by impacts with these accelerated electrons. In this conventional chemical ionization ion source, a differential evacuation should be conducted between the ionization chamber 1 and the electron gun 7, and they should be partitioned in a vacuum by an electrode having a slit (in general, 0.025-0.05 mm.times.3-5 mm in size). Because of the presence of this slit, introduction of electrons emitted from the electron gun 7 into the ionization chamber 1 is limited, and hence, the efficiency of utilization of generated electrons is very low. Further, since a heating filament is ordinarily used as the electron gun 7, the sample is thermally decomposed and the ionization chamber or slit is readily contaminated. This is another defect of the conventional ion source. Moreover, this contamination of the slit results in unstable currents of electrons introduced in the ionization chamber and consequently in instability of the quantity of produced ions. Still further, in the case where an electron source including a heating element is employed, if a corrosive gas is used as the reagent gas, the filament is damaged. Accordingly, it is not desired to use a corrosive gas such as O.sub.2 or H.sub.2 O. Furthermore, in order to measure the pressure of the ionization chamber 1 in an ion source having the above structure, it is necessary to dispose therein a vacuum gauge such as a MacLeod gauge. Hence, the capacity of the ionization chamber 1 must be increased. If the dead volume of the ion source is large, troubles are caused in the practical operation. For example, the injected sample is hardly taken out from the ion source but is left for an indefinite time. Therefore, provision of such vacuum gauge in the chemical ionization ion source is not preferred.